In recent years, with the rapid spread of various information devices and communication devices, such as personal computers, video cameras, and mobile phones, great emphasis has been put on developments of batteries as the power sources for such devices. In the automotive industries and other related industries, too, developments have been conducted for high-output and large-capacity batteries for electric vehicles and hybrid vehicles. Among such batteries under development, lithium batteries have been attracting much attention due to their high energy density.
In the lithium batteries presently available in market, an organic electrolyte solution containing a combustible organic solvent is used, and it requires various safety devices for preventing a temperature increase upon short-circuits and requires various improvements of the structure and materials for preventing short-circuits.
On the other hand, solid-state lithium batteries using solid electrolyte in place of liquid electrolyte do not contain any combustible organic solvent, and therefore the aforementioned safety devices and safety structures can be omitted or simplified, which is desirable in view of the production cost and the manufacturability. However, in a case where a sulfide-based solid electrolyte membrane is used as a solid electrolyte membrane, if the sulfide-based solid electrolyte membrane contacts the moisture in the air, sulfide hydrogen is produced and it causes significant deterioration of the performance of the sulfide-based solid electrolyte membrane.
In view of such problems, solid-state batteries have been proposed in which a power generation element constituted of a cathode, a sulfide-based solid electrolyte membrane, and an anode is sealed using sealant. As one of such solid-state batteries, a fully solid-state battery is described in Japanese Patent Application Publication No. 06-275274 (JP-A-06-275247) in which the power generation element is sealed by a high-temperature curing resin sealant. According to this solid-state battery, because the power generation element is protected by the sealant, the moisture in the air which causes degradation of the sulfide-based electrolyte membrane does not enter the power generation element, and therefore said degradation of the sulfide-based solid electrolyte membrane due to entrance of moisture into the power generation element does not occur. However, when Li ions move between the cathode and the anode upon power charge, power discharge, and so on, the volume of the power generation element changes (the power generation element expands or contracts), the portions sealed by the high-temperature curing resin may crack. In this case, the moisture in the air enters the power generation element via the crack and it causes degradation of the sulfide-based solid electrolyte membrane.